In known air conveyors for use in delivering a gas through a test tube, the gas to be analyzed is drawn through the gas test tube by means of a suction element in the form of a rubber ball pump. The air escapes to the outside through an air outlet valve when the rubber ball is compressed. On the other hand, when the rubber ball is released or permitted to decompress, air is drawn in through the gas analyzing test tube. This known air conveyor, however, has the disadvantage that the quantity of gas conveyed differs from one actuation of the rubber ball to the other, because the volume is a function of the various ways in which the rubber ball is compressed. Furthermore, the suction curve of the rubber ball has a discontinuity, at which a large amount of air is drawn in.
Another type of air conveyor for gas test tubes is disclosed in W. German Pat. No. 1,007,523, in which a bellows is used as the suction element instead of the rubber ball pump in order to reduce the discontinuity in the transport of air. The bellows are arranged between two holding plates, which must be pressed together by the hand of the operator against the action of two compression springs positioned in the bellows in order to empty the bellows and to prepare it for its suction phase. The disadvantage of this conveyor is that the two holding plates are not always pressed together completely or are pressed together only to a certain angle, with the result that the bellows will not be completely emptied. Consequently, in this type of air conveyor as well as the other, the amount of gas conveyed will differ from one actuation to another. There is the further disadvantage that the air conveyor is difficult to operate because it can slip out of the hand of the operator under the effect of the compression springs when it is operated with one hand.
The known air conveyors of the type just described must be actuated several times for one test, because the gas must be drawn several times through the gas tube. For that purpose, the air conveyors have a counter ring which is purely a marking to aid in indicating the number of strokes made with the rubber ball pump. This means that it is easy to make mistakes in the count and thus fail to execute the predetermined number of suction strokes. In the air conveyor of the bellows type, discussed above, it is known to include a stroke counter which is actuated when the two holding plates are compressed. However, because these plates are not positively guided in parallel, especially when they are operated by a single hand, it is possible that the counter is not actuated by the holding plate which makes the count. Furthermore, because of the rocking movements of the bellows, the stroke counter can be actuated even though a correct suction has not been accomplished, that is, the suction stroke actually executed does not correspond to the full suction volume of the bellows. And finally, especially when the device is operated by a single hand, it is possible for the suction stroke to be braked by the hand, because both holding plates must be gripped in the same hand.
It is crucial for the purpose of accurate measurement that the suction volume be the same for each stroke. This is not achieved either with the suction ball or with the known bellows--both are only approximations. In addition, the suction characteristic of each stroke, must be the same, which is not the case with the known air conveyors. For example, in the air conveyor according to W. German Pat. No. 1,007,523, the manual grip on the two holding plates must be completely released in order to release the bellows, so that the holding plates are not braked as they move apart. As soon as there is any braking action on the movement of the holding plates, the suction pressure is no longer constant, and the resulting measurement inaccurate. Finally, it is necessary for the amount of time which elapses during the suction stroke to be the same for each measurement. When the suction stroke is impeded through the incomplete release of the bellows, inaccurate measurements again occur.
Accordingly, it is an object of the invention to provide an air conveyor for gas test tubes of the type discussed above in which the air from the suction element is expelled in a complete, precisely defined, and nonmanipulatable manner before a suction stroke can be executed; in which counting errors are eliminated for the sequence of individual suction processes; and in which the suction element can be operated continuously and freely with a constant suction capacity, with identical suction characteristics, and with equal suction times.